Following his new team’s win over the Raptors on Tuesday night, Rockets guard James Harden was asked to peer ahead a night, to Wednesday’s trip to familiar terrain: Oklahoma City, where Harden played the first three years of his career and won the Sixth Man of the Year award last season, before an abrupt trade sent him out of town just before the season began. The Thunder’s reasoning for dealing Harden amid difficult contract negotiations was solid, but Harden has admitted he was hurt by the way the trade played out.

He wasn’t about to stir up controversy, though. “It’ll be good to see some faces, some old teammates and coaches,” Harden said. “I look forward to every game I play. It’s another game.”

Not to impugn Harden’s word, but that simply can’t be true—this is not just another game for him, it is a homecoming he had hoped would not be necessary, since he did not want to leave Oklahoma City in the first place. On some level it has got to sting that the Thunder, thanks in part to the 15.7 points and 49.3 percent 3-point shooting of his replacement, Kevin Martin, are sitting at 11-4, at their usual spot within a game of the Western Conference’s top spot. Houston, meanwhile, has been inconsistent, even as they are on a three-game winning streak that has them back at .500.

On the surface, not much has changed in Oklahoma City. The Thunder’s offense has been potent as always. They ranked second in overall offensive efficiency last year, scoring 0.967 points per possession according to Synergy Sports. They still rank second this year, bumping up their efficiency to 1.001 points per possession. The big difference in the increase has been their performance in transition, where they ranked sixth last year but are first this year.

But a deeper look at the numbers show where Harden is still desperately missed by the Thunder and why, when the playoffs come, his absence could be a big problem for OKC—Harden remains the most efficient pick-and-roll player in the NBA, and the Thunder have no one who can match him in that regard.

Last year, the Thunder were the best team in the league when it came to efficiency in plays in which the ballhandler in the pick-and-roll took a shot, averaging 0.894 points per possession. This year, they’re down to 0.654 points per possession, which ranks 27th. Overall, they run pick-and-rolls (including passes) 24.3 percent of the time, and their 0.918 points per possession rank 14th in the NBA. Last year, they ran pick-and-rolls 24.4 percent of the time and averaged 0.942 points per possession—good for fifth in the league.

In other words, on a play that the Thunder employ one-quarter of the time, they have gone from elite to average from this year to last.

The difference is obviously Harden. Last year, about a third of his offense came with him as the ballhandler in pick-and-roll situations, and he scored 316 points in 298 possessions, according to Synergy. His efficiency—1.06 points per possession—was easily the best in the NBA, with no one else who had more than 200 pick-and-rolls topping 1.0 points per possession. That hasn’t changed in Houston, where he is averaging 1.088 points per possession in the pick-and-roll. Only one other player (Kobe Bryant of the Lakers) is averaging better than 1.0 points per possession in the pick-and-roll.

No one on the Thunder can match that. Starting point guard Russell Westbrook was a good pick-and-roll player last year, with 0.837 points per possession, but now he has dropped to 0.71 points per possession, slightly below average in the league. Among starting point guards, Westbrook ranks 23rd in efficiency as the ballhandler in pick-and-roll situations.

That will hurt when the games toughen up in May and June, when fast-break points are harder to come by. In close, late-game situations, the pick-and-roll is an NBA staple. Harden is the best in the league at it, and though the Thunder are rolling without him, they will come to miss what he provided.